Pneumatic temperature measurement and control system



April 2, 1963 G. H. MESSERLY PNEUMATIC TEMPERATURE MEASUREMENT ANDCONTROL SYSTEM Filed Dec. 12, 1956 3 Sheets-Sheet 1 INVENTOR. iaa'i/lf/yzsjzizr April 2, 1963 e. H. MESSERLY 3,033,574

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PNEUMATIC TEMPERATURE MEASUREMENT AND CONTROL SYSTEM Filed Dec. 12, 19563 Sheets-Sheet 5 1 NV ENTOR.

United rates Fatent 3,083,574 PNEUMATIC TEWERATURE MEASUREMENT ANDCONTROL SYSTEM George H. Messerly, outh Bend, Ind, assignor to TheBendix Corporation, a corporation of Delaware Filed Dec. 12, 1956, Se N627349 9 Ciaims. {CL 73-357) This invention relates to temperaturesensing ad more particularly to a pneumatic means for measuringtemperatures and for providing an output signal which can be used in acontrol system.

In the field of fuel controls for gas turbine engines, ramjets, etc., itis often necessary or desirable to measure combustion temperatures whichmay approach the temerature limits of the metals in which the combustionprocess is confined. Thermocouples are often used for this purpose butthey have disadvantages in that they tend either to be slow in responseif the physical size is large enough to withstand the temperaturesinvolved, or if they are made of lightweight material capable of givingrapid response, they burn out after a short time and thus present aservicing problem. Further, the output signal of a thermocouple usuallyrequires amplification before it can be used to efiect a correction in asystem. Static gas thermometers wherein a gas is trapped inside abellows have been used but these systems tend to be slow in response andto require additional calibration to take into consideration the springrate of the bellows. Pneumatic thermometers have also been used in whichgases, the temperature of which it is desired to measure, are caused toflow through a tube having a plurality of restrictions therein, andcertain pressures are sensed to give an indication of the temperature ofthe gas. These systems have disadvantages because it is necessary tobring the extremely hot gases from a combustion chamber, directly intothe system. These combustion gases contain considerable components andsolids which upset calibration. It is, therefore, an object of thepresent invention to provide a pneumatic temperature sensor in which theproperties of the sensing fluid (no condensibles or solids) which flowsthrough the orifices is such that calibration is more precise than isthe case with pneumatic systems presently in use.

It is another object to provide a pneumatic temperature sensing systemin which inaccurate and calibration difficulties caused by bellows,spring rates etc., are at a minimum.

It is another object to provide a pneumatic temperature sensor which isdurable and capable of withstanding great extremes of temperature.

It is another object to provide a pneumatic temperature sensor in whichresponse times are sufiiciently short to enable it to be used to sensecombustion temperatures in engines and to provide a direct pneumaticsignal of sufficient energy that it may be used directly to cause acorrection in a control system.

It is another object to provide a temperature sensor which may be madein a very small and light package.

It is another object to provide a pneumatic temperature sensor whichwill retain its calibration despite some changes in volume of gaschambers.

It is a further object of the present invention to provide a temperatureresponsive system in which the output may be any of a number ofarbitrary functions of temperature.

Other objects and advantages will become apparent from the followingdescription taken in connection with the following drawings, in which:

FIGURE 1 is a schematic drawing illustrating generally the principle ofmy invention;

FIGURE 2 is a schematic drawing of one form of my "inc invention inwhich compensating means are provided for changing inlet temperatures;

FIGURE 3 is a schematic drawing illustrating a modified form of myinvention;

FIGURE 4 is a schematic drawing showing an alternate arrangement of myinvention;

FIGURE 5 is a schematic drawing illustrating a modification of thedevice of FIGURE 2 to embody the gas flow relationships of FIGURE 4;

FIGURE 6 is a schematic drawing of a modification of the device ofFIGURE 1; and

FIGURE 7 is a schematic drawing illustrating a modification of thedevice of FIGURE 2 to embody the gas flow relationships of FIGURE 6.

Referring to FIGURE 1, gas flow is directed through two conduits 10 and12 arranged in parallel. Conduit 19 has a pair of restrictions =14, 16in series and conduit 12 has a pair of restrictions 18, 20 in series.The gas supplied to the conduits 10 and 12 is from a common source at apressure P This pressure is preferably high enough so that flow throughall of the restrictions is sonic; i.e. the pressure ratio across eachorifice is equal to or greater than approximately two. The gas flowthrough restrictions 14, 18 and 24 is maintained at a constant knowntemperature T Flow through restriction 16 is maintained at thetemperature which it is desired to measure, T Enough length of conduit10 upstream of restriction 16 must be exposed to temperature T to causethe temperature of the gas flowing through the conduit to be stabilizedat temperature T With the arrangement of FIGURE 1, temperature T may bedetermined as shown below. For sonic flow conditions, the gas flowthrough an orifice is defined by the following equation:

where W=weight of gas C: a constant A area of orifice P=pressureupstream of the orifice T: absolute temperature of the gas The fiowthrough orifices 14 and 16 is the same, therefore W W Where Where C FFsolving for P is as follows:

3 l6 T1 14 2 T2 The flow through orifices 18 and 20 is the same niannw vT1 v T1 where C =constant associated with restriction 18 C =constautassociated with restriction Z0 P =pressure upstream of restriction 18 P=pressure upstream of restriction 20 T =absolute temperature of gas flowthrough restrictions 18 and 20.

where 05 v r C; a,

solving for P is as follows:

C6AZOP2II 18 Therefore,

7 C3A16P2 T 1=C6A2OP2, 14 E n Because the areas of orifices 14, 1'6, '18and 20 can be made such that and because 'the constants, which areessentially the discharge coeificients of the orifices would be thesame, these factors will cancel leaving 12': 7 T; P; T;

' then duit 11 which branches into a conduit and a conduit I 12. Gasflow in conduit 10 passes through a restriction 1'4 and, while flowingthrough a heat exchanger 15 which is actually part of conduit 10 it israised to temperature T before flowing through restriction 16. Gas incon- 7 duit 12 flows through a variable restriction 18 and a variablerestriction 20. Flow through all restrictions is sonic. Connected toconduits 10 and 12 downstream of restrictions 14 and 18, respectively,is a pressure ratio sensing device 22 consistingof 'a'h'ous'in'g 24separated into two chambers 26 and 28 by means of a diaphragm 30. Thepressure in chamber 215 is P pressure and varies with changes in T asset forth above. Chamber 28 contains, gas at T temperature and Ppressure. .The difference between pressures (P -P is sensed by thediaphragm 30 which assumes a position in housing 24 such that P P Itwill be remembered that according to the mathematical relation set forthabove the temperature sensed varies with I i /P 'However, where apressure difierence system is used 'such that P P the position of thediaphragm will be the same as in the case of a ratiometer stabilizedsuch that P /P "=1. A rod 32 connected to diaphragm 30 carries acontoured needle 34 for varying the efliective area of orifice 1-8, anda pointer 36 which provides a reading on a temperature scale 38. Withproper contouring of needle 34, the scale 38 may be read directly interms of temperature T As indicated above, the device of FIGURE 1 wasbased on the presumption that T was known and constant. In practicalsituations there are many cases 'where assumption cannot be made valid,there being no convenient source of constant reference temperature.Changes in T temperature may be compensated for by 7 making the Ppressure vary with changing T If the area of restriction is madevariable by means of a contoured valve member 40, the contour may bemade an arbitrary function of T Movement of member 40 may be controlledthrough the action of any of a number of fled. Where T is system as afactor of unity,

differential thermal expansion devices. In the present instance, thishas been shown as a series of bi-metallic discs 42 which expand andcontracts with changes in T thereby moving member 40 such that pressureP is caused to vary with T 1 In FIGURE 3 is shown a system similar.tothat of FIGURES 1 and 2 wherein the structure is greatly simpli knownand constant and F is regulated to a known constant value, it ispossible to a single conduit 10 having restrictions 14 and 16' inseries, It is further required that the temperature of the gas flowthrough restriction 16 be stabilized at temperature T 'When flow throughboth orifices is sonic,

If P is regulated to be equal to C T or if T and P are held constant andA /A is a constant built into the i then T varies directly with PTherefore, a pressure gauge 44 responsive to P may be calibrated interms of T In FIGURE 4 is shown an alternate arrangement which resultsin a particularly convenient relationship between the intermediatepressures in the pneumatic circuits and! the absolute temperature ratio.In this case, the gas now through the upper conduit, to which will beassigned} numeral 50, is the same as with conduit '10 of FIGURE 1; Le,flow is from a source at a pressure P; and a known reference temperatureT through a restriction 52 and then through enough length of conduit sothat gas flow through the second restriction in series 54 is stabilizedat the unknown temperature T Flow through the other conduit 56 is fromthe high pressure source P; such that i the gas temperature reaches Tbefore flowing throagh the first orifice, 58. This flow then continuesthrough the conduit 56 until it enters a portion of said conduit attemperature T This portion is sufliciently long that now through thesecond orifice, 60 is stabilized at, temperature 7 T If the intermediatepressures in conduits '50 and 56 are assigned values of P and P',f1'es'pectively, the resulting relationships are as set forth below: vIn conduit 50, the flow rel'ationship is eiractly the same as in conduit10 of FIGURE 1. Weight of gas flow then,

is as set forth below:

W V 4T1 VTZ solving for P 7 A F P =0 P( l 1 0 2 A52 :112

In conduit 56 flow is as set forth below:

P1 7 5s) 2' so) therefore,

1 152 TI H158v T1 K n ss T1 The areas of orifices 52, 54, 58 and 60 maybe calibrated so that the above area factor is 1.

Therefore,

If T is known, then T is directly proportional to the pressure ratio P/P An instrument similar to that shown in FIGURE 2 is used in FIGURE 5,except that it utilizes the flow relationship of FIGURE 4. Gas underpressure P sufficient to cause sonic flow through all the orifices issupplied to a conduit 51 which branches into conduits 50 and 56. Flowthrough conduit 50 is directed through two restrictions in series, arestriction 52 through which flow is maintained at temperature T and arestriction 54 through which flow at temperature T is directed, saidtemperature being established by means of a'heat exchanger 55. Flowthrough conduit 56 is first into the T region where the gas isstabilized at T temperature by means of a heat exchanger 57 before beingsupplied to restriction 58, thence another heat exchanger 59 attemperature T before flowing through restriction 60. Connected toconduits 50 and 56 downstream of restrictions 52 and 58, respectively,is the pressure sensing device 22 which is exactly the same as thatshown in FIGURE 2. Movement of the diaphragm 30 causes movement of rod32 with changes in the pressures P and P Rod 32 carries a contouredneedle 34 for varying the efiective area of restriction 58, and apointer 36 which provides a reading on a temperature scale 38. In thiscase if the T scale is linear the contour of needle 34 must provide forlinear travel of rod 32 with P P Compensation for changing T is providedas in the FIGURE 2 device, through the action of a contoured needle 40which varies the effective area of restriction 60 with changes in theposition of the movable end of bellows 42 which is responsive to changesin T; as previously discussed.

If it is required to measure the temperature of a gas which is itself ata sufiiciently high pressure, and can conveniently be tapped for thepurpose, the arrangement shown in FIGURE 6 can be used. With thisarrangement the temperature and pressure relationships are analogous tothose of the devices of FIGURES 1 and 2 and the temperature can bemeasured by means of a device like that of FIGURE 7. The device ofFIGURE 7 is essentially the same as that of FIGURE 2 and the parts arenumbered accordingly. Gas at pressure P and temperature T is supplied toconduit 11 and thence to each of conduits 1t and 12. Flow throughrestriction 14 in conduit it) is maintained at T and is then allowed tocool to temperature T before flowing through restriction 16. Temperatureof the flow through conduit 12 is allowed to cool, such that flowthrough both of restrictions 18 and 24 is at temperature T The diaphragm30 then senses the pressure difference as described above and, if thecentral valve member 34 and the restriction 18 are calibrated as inFIGURE 2, movement of rod 32 and, hence, pointer 35 should be linearwith (Pg/P and scale 38 can be given a linear calibration in terms oftemperature. The bellows 42 acts to vary the position of needle 40 withchanges in temperature T as described above.

The above system, by using the gasses, e.g., combustion gasses, directlyavoids the requirement of supplying a separately introduced gas andbringing it into thermal equilibrium at temperature T However, it hasthe disadvantage that the gas properties (condensation, specific heatratio) may vary, causing uncompensated errors; that the pressureavailable in the T zone may not be favorable, and that the calibrationmay be upset by the condensation of moisture or the presence of solidparticles.

While a number of embodiments have been shown, it will be recognizedthat any one of these may be made in any of several forms depending uponthe requirements of a particular application. It will be apparent fromthe foregoing that the pressure function which it is ultimately desiredto utilize is the ratio P '/P however, where the system is used as partof an over-all control system it is sometimes more convenient to utilizea pressure sensor which provides an output varying with P P as shown anddescribed herein. This is especially true where the temperature signalis used simply to provide an overriding function when temperatures inexcess of a certain value are sensed. One reason for this is thatpressure difference sensing structure is usually simpler and lesssubject to inaccuracies than are pressure ratio sensors.

I claim:

1. An instrument for the measurement of a temperature comprising a firstconduit connected to a source of gas under pressure having a pair offixed area restrictions in series, said first conduit being exposed overa portion of its length to the temperature to be measured and overanother portion of its length to a known reference temperature, one ofsaid restrictions being located in said portion exposed to the measuredtemperature, said portion acting to stabilize the temperature of the gasflow through said restriction at said temperature to be measured, theother of said restrictions being located in said other portion exposedto said known reference temperature, said other portion acting tomaintain the temperature of the gas flow through said other restrictionat said known reference temperature, a second conduit connected to saidsource of gas under pressure having a pair of restrictions in series, apressure responsive device, first and second passages communicating saidpressure responsive device with said first and second conduits,respectively, at a point between the respective pair of seriesrestrictions contained therein, said pressure responsive device beingresponsive to the differential between gas pressures existing betweeneach of said pair of restrictions in series, which pressure differentialvaries as a predetermined function of said measured temperature, meansoperatively connected to said pressure responsive device and one of therestrictions in said second conduit for varying the effective area ofsaid one restriction in response to said pressure differential, andmeans responsive to variations in said known reference temperatureoperatively connected to the other of said restrictions in said secondconduit for varying the effective area of said other restriction as afunction of said known reference temperature.

2. An instrument for the measurement of a temperature as set forth inclaim 1 wherein said second conduit is exposed over a portion of itslength to the temperature to be measured and said one of therestrictions in said second conduit is also located in said portion ofsaid second conduit, said portion acting to stabilize the temperature ofthe gas flow through said one restriction at the temperature which it isdesired to measure.

3. An instrument for the measurement of a temperature as set forth inclaim 1 wherein said means for varying the effective area of said onerestriction in said second conduit includes a valve member positionedlinearly as an arbitrary function of said pressure differential and thussaid measured temperature.

4. An instrument for the measurement of a temperature as claimed inclaim 1 wherein said second conduit is exposed over a portion of itslength to said known reference temperature and said one of therestrictions in said second conduit is also located in said portion ofsaid second conduit, said portion acting to stabilize the temperature ofthe gas flow through said one restriction at said known referencetemperature.

5. An instrument for the measurement of a temperaure as claimed in claim1 wherein said second conduit is exposed over a first portion of itslength to said temperature to be measured and over a second portion ofits length to said known reference temperature, said first portioncontaining said one restriction in said second conduit such that the gasflow therethrough is stabilized at said temperatu'reto be measured, saidsecond portion containing other restriction in said second conduit such.that the gasflowdherethrough is stabilized atsaid knownreference-temperature;

' 6. instrument for the measurement of a temperature as claimed inaclaim 1 wherein said, one restriction in said first conduitis upstreamfrom said first passage and said other restriction in said first conduitis downstream from said first passage.

7. An instrument for the measurement of a temperature as'xclaimed inclaim ;1 wherein said one restriction in said first conduit isdownstream from said first passage and said other restriction in saidfirst conduit is upstream from said first passage.

.8. "An instrument for the measurementof a tempera-f ture comprising afirst conduit connected to a source of gas underpressure having a pairof fixed area restrictions inseries, said'conduit beingexposed to thetemperature to hemeasureduand tova known reference temperature, saidrestrictions being positioned in said conduit such that the gas flowthrough one is stabilized at the temperature to be" measured and the gasflow through the other is maintained at said reference temperature, saidgas flow through said .one restriction varying as a predeterminedfunction of the temperature. to be measured, a second conduit connectedto said source of gas under pressure having two restrictions in series,said second conduit being exposed tosaid reference temperature, saidsecond conduit acting to maintain the temperature of the gas flowthrough said two restrictionsrin saidsecond conduit at the referencetemperature,- a pressure measuringdevice connected to both of saidconduitsand responsive to the gas pressures existing between each ofsaid pair of restrictions in series,

which pressure relationship varies as a predetermined function of saidmeasured temperature, and means including a valve member operativelyconnected to said measurv the temperature to be measured, saidsecondconduit having two restrictions in series and being exposed to a knowntemperature such that the gas flow through said two restrictions thereinis maintained at said known temperature, and apressure sensing deviceconnected 'tosaid' first conduit between it'stwo said restrictions andto said second' conduit'between' its two said restrictions, the ratio ofsaid pressures being a known function ofsaid temperature to" be measured7 I 1 References Cited in the, file of this patent UNITED A E PAT NT 7ore Au FOREIGN PATENTS France Jan. '7,

9. AN INSTRUMENT FOR THE MEASUREMENT OF A TEMPERATURE COMPRISING FIRSTAND SECOND CONDUITS, BOTH OF WHICH ARE SUPPLIED FROM A SOURCE OF GAS ATA TEMPERATURE TO BE MEASURED, SAID FIRST CONDUIT HAVING TWO FIXED AREARESTRICTIONS IN SERIES AND THE FIRST OF SAID RESTRICTIONS BEING SOLOCATED THAT THE GAS FLOWING THERETHROUGH IS STABILIZED AT THETEMPERATURE TO BE MEASURED, SAID SECOND CONDUIT HAVING TWO RESTRICTIONSIN SERIES AND BEING EXPOSED TO A KNOWN TEMPERATURE SUCH THAT THE GASFLOW THROUGH SAID TWO RESTRICTIONS THEREIN IS MAINTAINED AT SAID KNOWNTEMPERATURE, AND A PRESSURE SENSING DEVICE CONNECTED TO SAID FIRSTCONDUIT BETWEEN ITS TWO SAID RESTRICTIONS AND TO SAID SECOND CONDUITBETWEEN ITS TWO SAID RESTRICTIONS, THE RATIO OF SAID PRESSURES BEING AKNOWN FUNCTION OF SAID TEMPERATURE TO BE MEASURED.